The invention is in the field of electronic reproduction technology and is directed to a method for generating a contone map for electronic exposers or recorders with high resolution for pixel-by-pixel and line-by-line exposure of recording material.
In reproduction technology, printer's copies are produced for printed pages that contain all elements such as texts, graphics and images to be printed. FIG. 1 shows an example of a printed page. A separate printer's copy that contains all elements that are printed in the respective color is produced for each ink in chromatic printing. These are the inks cyan, magenta, yellow and black (C,M,Y,K) for four-color printing. The printer's copies separated according to inks are also called color separations. The printer's copies are usually screened and exposed in high resolution on films that are then further-processed for producing the printing forms (printing plates, printing cylinders). Alternatively, the printer's copies can also be directly exposed onto printing plates in special recorders. For reviewing the content and the colors of the printed pages, printer's copies are exposed in proof recorders with a recording process that simulates the printing process in a chromatic output. Instead of individual printed pages, printer's copies can also contain signatures that are composed of an arrangement of a plurality of printed pages.
FIG. 2 shows the work sequence in the exposure of printer's copies for printed pages produced in the page description language PostScript that has been mainly employed in the prior art up to now. The PostScript data 1 are supplied to a raster image processor (=RIP) (2), which can be a computer specifically optimized for this job or a program on a standard computer. PostScript data 1 for every color separation are normally generated in a pre-process for every color separation of a printed page and are forwarded to the RIP (2) (separated PostScript). Alternatively, a chromatic printed page can also be generated in a single PostScript dataset (composite PostScript). The case of separated PostScript data 1 shall be explained in greater detail below.
In a first step, the PostScript data 1 are analyzed in an interpreter 3 and resolved into a sequence of simple graphic objects. For that purpose, the printer's copy is divided into horizontal strips (bands) that are successively processed. FIG. 3 shows a band excerpt 9 with a few objects generated by the interpreter. The band excerpt 9 is divided into recording pixels 10. In the example of FIG. 3, the band excerpt is 8 pixels high, numbered from 0 to 7, and 32 pixels wide, numbered from 0 to 31. The resolution can be symmetrical (the same in horizontal and vertical direction) or asymmetrical, for example twice as great horizontally as vertically. The objects A through E (11,12,13,14,15) describe sub-segments of text, graphics or image elements that fall within the band excerpt 9.
The interpreter outputs the objects A through E (11,12,13,14,15) in a data format that is referred to as display list 4 (FIG. 2). For each object, the data format describes its geometrical shape and the gray scale value with which it is filled. The objects A through E (11,12,13,14,15) appear successively in the display list 4 in the sequence in which the corresponding page elements are described in the PostScript data. Objects that appear later in the display list 4 can thereby partly or entirely cover objects that appeared earlier in the display list 4. In the example of FIG. 3, the object A (11) is partly covered by the object B (12). Likewise, the objects D (14) and E (15) cover the object (C).
In a further step in the RIP 2, the display list 4 is supplied to a raster generator 5 that successively converts the objects of the display list 4 into AREAS filled with raster points and enters them into a bit map memory 7 as bit map data 6. The raster point size is thereby varied dependent on the gray scale value of the object in the display list 4. The bit map data 6 of objects that appear later in the display list 4 respectively overwrite the corresponding of areas of the bit map memory 7. After all objects of a band have been rastered by the raster generator 5 and written into the bit map memory (7), the content of the bit map memory (7) is forwarded as control signal values to the recorder (8) and exposed thereat.
Due to the overlap of the objects in the display list 4 and the repeated screening of sub-areas in the bit map memory 7 that overlap, the time that the RIP 2 requires in the previous procedure for the processing and output of a band to the recorder 8 is variable and not predictable. It is dependent on how many objects occur in a band and the proportion to which they overlap. Given the high speed of modern recorders, the data rate upon utilization of an asymmetrical resolution can, for example, amount to up to 200 million pixels per second. If it is not assured that the RIP 2 can continuously supply the control signal values for the recorder 8 with the data rate prescribed by the recorder speed, then the recorder 8 must operate in what is referred to as a start/stop mode. In the start/stop mode of a recorder, the exposure is interrupted given the lack of the control signal values until the RIP 2 again supplies control signal values, and the exposure is then seamlessly continued at the location of the interruption.
The mechanical and optical design of a recorder that can expose films or printing plates in start/stop mode with high resolution without having the start/stop locations visible in the finished recording is more complicated and costly than for a recorder that exposes continuously. The stopping and reactivation of the recorder, moreover, requires additional time for each start/stop event, so that the exposing can thereby last considerably longer than given a continuous operation of the recorder.
On the other hand, a RIP that, regardless of the complexity of the page contents, can respectively convert the PostScript data for printed pages into bit map data so fast that it can always keep step with the exposure speed of a continuously operating exposer requires extremely fast processors and large memories and thereby likewise becomes expensive.
The solution of making the bit map memory 7 in the RIP so large that it can intermediately store the bit map data of an entire printer's copy (page buffer) is not practical since the memory then becomes extremely large and costly. For a printing plate having the size 70 cm.times.100 cm and a resolution of 2666 pixels/cm (6772 dpi; dpi=dots per inch) horizontally and 1333 lines/cm (3383 dpi) vertically, a buffer size of 3109.6 Mbyte derives. A hard disk is eliminated as a page buffer since it cannot read the bit map data out with the required speed of 100 to 200 Mbit/s.
As a result of the fact that some commercially available recorders offer no possibility or only an inadequate possibility of a start/stop mode, there is a necessity of achieving an adequately high speed in the data conversion in the RIP with reasonable processor and memory costs.
As a consequence of the high memory requirement for the finished bit map of a printer's copy, the bit map of a printer's copy can usually not be intermediately stored given the previous procedure for the exposure of PostScript data. When the same printer's copy is to be exposed again, for example because the film or the printing plate exposed first was damaged, the entire processing procedure from the interpretation of the PostScript data up to the exposure must be repeated. This costs additional time and occupies the RIP that could already process a new printer's copy during this time.
For the same reason, the additional exposure of the printer's copy on a proof output unit--given the previous procedure--requires another entire run of the PostScript data through the RIP and therefore wastes unnecessary time. Added thereto as a further disadvantage is that the proof device generally has a different resolution than the film exposer for color separations, so that the RIP must work with different resolution for the two output units. This leads to minute differences between the proof output and the film output, for example in regions where objects exactly adjoin one another. The proof output therefore does not always provide all of the details of the later print. This runs contrary to the purpose of a proof output and is therefore disadvantageous.